Scale-up studies of Microbial Fuel Cells are required before practical application comes into sight. We studied an MFC with a surface area of 0.5 m2 and a volume of 5 L. Ferric iron (Fe3+) was used as the electron acceptor to improve cathode performance. MFC performance increased in time as a combined result of microbial growth at the bio-anode, increase in iron concentration from 1 g L-1 to 6 g L-1, and increased activity of the iron oxidizers to regenerate ferric iron. Finally, a power density of 2.0 W m-2 (200 W m-3) was obtained. Analysis of internal resistances showed that anode resistance decreased from 109 to 7 mO m2, while cathode resistance decreased from 939 to 85 mO m2. The cathode was the main limiting factor, contributing to 58% of the total internal resistance. Maximum energy efficiency of the MFC was 41%.